1. Field of the Invention
This invention relates to a circuit for evaluation of a measurement signal of a thermocouple, especially a thermocouple which is used as a flame detector of a motor vehicle heater, with an amplifier for amplifying the measurement signal and with a microcontroller which is supplied with the amplified measurement signal as the input signal.
2. Description of Related Art
In the evaluation of thermocouple measurement signals, typically a series of problems rise. To make it possible to evaluate these measurement signals with a voltage in the millivolt range by a microcontroller, they must be amplified into the volt range. Based on the required evaluation accuracy, expensive amplifiers, specifically measuring amplifiers, especially in the form of integrated components, are ordinarily used for this purposes.
The problem in the evaluation of thermocouple measurement signals is, furthermore, that a defect of the thermocouple, for example, its interruption, cannot be detected without higher circuit cost. Another problem is the fact that the thermocouple measurement signal, depending on the temperature difference between the thermocouple and the control device or amplifier, has a positive or negative sign, while the microcontroller can typically process only positive voltages. Finally, on principle, only temperature differences between two points can be measured with a thermocouple. To determine the absolute temperature by means of a thermocouple, therefore additional, conventionally complex measures must be taken.
In view of the foregoing, a primary object of this invention is to devise a circuit for evaluating a measurement signal of a thermocouple of the initially mentioned type which ensures the required accuracy using economical components.
This object is achieved in a circuit for evaluation of a measurement signal of a thermocouple, especially of a thermocouple which is used as a flame detector of a motor vehicle heater, with an amplifier for amplifying the measurement signal and with a microcontroller which is supplied with the amplified measurement signal as the input signal, by there being a switch for short circuiting the input of the amplifier during a pause in the measurement cycle, the microcontroller having a memory in which the input signal is deposited during the measurement cycle pause, and the microcontroller having a correction algorithm to correct the measurement signal using the signal deposited in the memory before evaluation.
In other words, the invention calls for acquisition of construction-induced errors of the amplifier, which acquired errors are thereupon taken into account in the microcontroller in the evaluation of the thermocouple measurement signal, so that a measurement signal which has been corrected with respect to amplifier errors is evaluated. The measures in accordance with the invention thus allow use of an economical, faulty amplifier for amplification of the thermocouple measurement signal instead of the expensive measuring amplifiers which have been used in the past. Typically, an economical, less precise standard operational amplifier costs much less than, for example, roughly only one tenth as much, a high precision measuring amplifier. The measures in accordance with the invention, for error correction, i.e., the components and programming measures used for this in the microcontroller increase the costs only slightly, so that the circuit according to the invention is, in any case, much more economical than those existing heretofore. Furthermore, it has been ascertained that the accuracy which can be achieved by the error correction of the invention, when using an economical amplifier, ensures a reliable conclusion about the temperature prevailing at the measurement site without recourse to the accuracy made available by a measuring amplifier.
The amplifier of the inventive evaluation circuit is advantageously connected as a difference amplifier with a noninverting input and an inverting input which are connected to two terminals of the thermocouple and are bridged by a switch, by the closing of which pauses the measurement cycle, the voltage inherent in the absence of an amplifier being relayed to the microcontroller for subsequent error correction of the measurement signal.
The switch is preferably a semiconductor switch, advantageously in the form of a field effect transistor (FET). In the case of connecting the amplifier as a difference amplifier, this switch is connected between its noninverting input and its inverting input.
The switch is activated advantageously via the control output of the microcontroller which carries the measurement cycle.
In order to economically and simply overcome the initially addressed problem of detection of a thermocouple defect, according to one advantageous development of the invention, there is additionally a circuit for supplying a test current to the thermocouple during a pause in the measurement cycle, and the microcontroller comprises a means for evaluating its input signal which is present in the pause of the measurement cycle. Preferably, this circuit is coupled on the input side to the amplifier such that no noticeable voltage is produced at the amplifier output by the test current when the thermocouple is working properly, while when the thermocouple is defective, an indicative voltage which differs from the previous one occurs at the amplifier output.
The test current for detecting the thermocouple state is preferably sent to the thermocouple by means of a switch which, in turn, is designed advantageously as a semiconductor switch, for example, as a FET, and is triggered via the control output of the microcontroller, i.e., the output which carries the measurement cycle.
According to an advantageous development of the invention, the initially mentioned problem, according to which a microcontroller can process typically only positive voltages, is surmounted, in accordance with the invention, by a positive DC voltage being added to the amplified measurement signal, the DC voltage being higher than the amount of the negative measurement signal voltage at the addition point. This ensures that the voltage at the microcontroller input is always in the positive range.
Finally, a measure is in accordance with the invention to solve the initially mentioned problem of acquiring the absolute temperature by means of the thermocouple. Accordingly, there is a means for measuring the absolute temperature of the board which bears the amplifier and the temperature difference between the measurement point of the thermocouple and the board, from which the microcontroller computes the absolute temperature of the measurement point.
In the following, a preferred embodiment of the invention is explained using the drawings.